Now investigators believe the cause may have been a pet store purchase gone bad.

"It was the wrong kitty litter," says James Conca, a geochemist in Richland, Wash., who has spent decades in the nuclear waste business.

It turns out there's more to cat litter than you think. It can soak up urine, but it's just as good at absorbing radioactive material.

"It actually works well both in the home litter box as well as the radiochemistry laboratory," says Conca, who is not directly involved in the current investigation.

Cat litter has been used for years to dispose of nuclear waste. Dump it into a drum of sludge and it will stabilize volatile radioactive chemicals. The litter prevents it from reacting with the environment.

And this is what contractors at Los Alamos National Laboratory were doing as they packed Cold War-era waste for shipment to the dump. But at some point, they decided to make a switch, from clay to organic.

"Now that might sound nice, you're trying to be green and all that, but the organic kitty litters are organic," says Conca. Organic litter is made of plant material, which is full of chemical compounds that can react with the nuclear waste.

"They actually are just fuel, and so they're the wrong thing to add," he says. Investigators now believe the litter and waste caused the drum to slowly heat up "sort of like a slow burn charcoal briquette instead of an actual bomb."

After it arrived at the dump, it burst.

"How come nobody caught this and raised a red flag?" asks Ryan Flynn, New Mexico's secretary of the environment. Flynn says that the cause of the accident still isn't 100 percent clear. Scientists at Los Alamos have yet to find the exact blend of cat litter and nuclear waste that can spark a reaction.

But he says it is clear that the wrong material went into some of the drums. He wants to know why the Department of Energy, which handles the waste, let this happen.

"I'm frustrated," he says.

Flynn says there are more than 500 drums packed with the wrong litter. The majority are relatively safely underground in the dump, but dozens are still at Los Alamos and another site in West Texas. None of these drums have burst so far, but the lab and the company handling the Texas waste have put them in heavy containers for added protection.

Flynn says federal authorities need to come up with a long-term solution and prevent future mix-ups.

"Ultimately [the waste is] the responsibility of the Department of Energy," he says. "It's also now their responsibility to clean it up and fix it."

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Transcript

AUDIE CORNISH, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish.

ROBERT SIEGEL, HOST:

And I'm Robert Siegel. Around the newsroom, we've been calling this next item the nuclear kitty litter story. It refers to a case in which the government may not have been watching things closely enough. In February, an accident occurred at the only nuclear waste dump in the U.S., the Waste Isolation Pilot Plant in New Mexico. A 55-gallon drum burst releasing its radioactive contents.

No one was hurt, but the dump has remained closed ever since. NPR's Geoff Brumfiel reports that investigators now believe the cause was human error, lax oversight and a bad pet store purchase.

GEOFF BRUMFIEL, BYLINE: If I told you the leading theory about what caused this nuclear accident, you wouldn't even believe me. So here's James Conca, a PhD geochemist.

JAMES CONCA: It was the wrong kitty litter.

BRUMFIEL: The wrong kitty litter.

UNIDENTIFIED WOMAN: There's another way to help erase litter box odor.

BRUMFIEL: It turns out there's more to kitty litter than you think. It can soak up cat pee, but it's just as good at absorbing radioactive material.

CONCA: People don't realize the science that went into developing kitty litter. I mean, it was a really big thing.

BRUMFIEL: Kitty litter has been used for years to help dispose of nuclear waste. Dump it into a drum of radioactive sludge and it will stabilize it. The kitty litter soaks up chemicals. And this is what contractors at Los Alamos National Laboratory were doing as they packed Cold War-era waste for shipment to the dump. But at some point, they decided to make a switch, from clay to organic kitty litter.

CONCA: Now that might sound nice, you're trying to be green and all that, but the organic kitty litters are organic.

BRUMFIEL: It's made of plant material, which is full of chemical compounds that can react with the nuclear waste.

CONCA: They actually are just fuel and so they're essentially the wrong thing to add.

BRUMFIEL: The leading theory about this accident is now this. The organic kitty litter cause the drum of waste to slowly heat up.

CONCA: Sort of like a slow burn charcoal briquette.

BRUMFIEL: After it arrived at the dump, it burst.

RYAN FLYNN: How come nobody caught this and raised a red flag?

BRUMFIEL: That's Ryan Flynn, New Mexico's secretary of the environment. The Department of Energy oversees Los Alamos' nuclear dump. Flynn says it should have been aware of the switch and stopped it.

FLYNN: You know, I'm frustrated.

BRUMFIEL: Scientists at Los Alamos are now trying to recreate the exact blend of kitty litter and nuclear waste that sparked the reaction. Flynn wishes he was talking to NPR about something else.

FLYNN: In my dream, I was going to be talking to Sylvia Poggioli commenting on some really cool Italian village or doing something other than talking about kitty litter.

BRUMFIEL: According to him, there are more than 500 drums packed with the wrong kitty litter. The majority are in an underground dump, but dozens are still at Los Alamos and another site in West Texas. None of these drums have burst so far, but the lab and the company handling the Texas waste have put them in heavy containers for added protection.

Flynn says federal authorities need to come up with a long-term solution and prevent future mix-ups.

FLYNN: Well, ultimately it's the responsibility of the Department of Energy and it's also their responsibility to now clean it up and fix it.

BRUMFIEL: Until the exploding kitty litter problem is solved, all shipments to the New Mexico dump will remain on hold. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.